Calorie restriction (CR), without malnutrition, is the only intervention that has consistently been shown to increase maximum life span and prevent or delay the onset of age-associated pathophysiological changes in laboratory rodents. It has been proposed that the degree of membrane fatty acid unsaturation is a major factor contributing to longevity, and that a decrease in membrane unsaturation may be the mechanism for the retardation of aging with CR. Such theories that link membranes to aging solely through modulation of membrane lipid peroxidizability, however, may be too general since they overlook the affect lipid alterations will also have on membrane-linked processes, such as reactive oxygen species (ROS) production and electron transport chain activity. Also, these theories ignore the role membrane proteins play in protecting against oxidative damage. Considering the central role that membranes play in regulating oxidative stress, we hypothesize that CR causes an alteration in plasma and mitochondria! membrane composition that results in a new bioenergetic balance leading to decreases in both ROS production and membrane oxidative damage. We propose three specific aims to test this theory: Specific Aim 1;To determine membrane composition (fatty acids, phospholipids and coenzyme Q) of mitochondrial and plasma membranes from liver and skeletal muscle of control and 40% CR C57BL/6 mice. Comprehensive lipid analysis will be completed to quantify all classes of phospholipids and their constituent fatty acids. Markers of membrane oxidative damage will also be measured. These studies will thoroughly quantify CR-induced changes in membrane lipid composition, and determine if these changes result in a decrease in membrane peroxidizability. Specific Aim 2;To determine if CR induces alterations in mitochondrial and plasma membrane-linked processes in liver and skeletal muscle from control and 40% CR C57BL/6 mice. Membrane-linked processes related to energy expenditure, ROS production, oxidative damage, and antioxidant defenses will be measured. Groups of CR mice will also be fed diets where the fat source is beef tallow (highly saturated), soybean oil (high linoleic acid), or menhaden fish oil (high n-3 polyunsaturated fatty acids) to determine if alterations in membrane fatty acid composition are required for CR-induced changes in mitochondrial and plasma membrane functions. Specific Aim 3: To determine if decreases in membrane long chain polyunsaturated fatty acids or increases in activity of the plasma membrane redox system are required for life span extension with CR. Genetic (fat-1 mice) and dietary interventions will be used to determine if CR-induced changes in n-3 fatty acids, linoleic acid, or membrane saturation are required for life span extension. Also, NAD(P)H-quinone reductase knockout (NQO1 KO) mice will be used to determine if stimulation of the PM redox system is required for life span extension with CR.